The selection is made by the number of credits in each section. You are right on the cusp of having your acting credits first.
On the Pro version of the pages, the default is by Production status, but when I select by Job, Actor is the first section listed. Since both Sound and Actor have 6 credits, it may be a matter of chance which shows first.
If you get one more Actor credit (and no new Sound credits) the acting credits will be moved to the top of the list.